The effects of surface microheterogeneity and micromorphology on blood materials interactions is largely unknown. The styrene-butadiene-styrene (SBS) triblock copolymer series permits the control of micromorphology and, by selective hydroxylation of the butadiene blocks, permits hydrophilic microdomains and hydrophobic microdomains to coexist side by side. By appropriate control of the fabrication conditions and the initial polymer composition the micromorphology can be varied over a wide range. It is proposed to control the micromorphology by appropriate processing conditions and copolymer composition and to study the effect of micromorphology on blood-materials interactions. Present cardiac assist device materials consist largely of block copolymer systems based on the polyetherurethanes. Though these materials have been relatively successful, there is increasing concern about their long-term stability in the blood and tissue environment, particularly due to lipid adsorption and long term calcification. Other elastomer systems may prove to be more stable and less prone to biological attack or degradation for long-term implants. The styrene-ethylene-butylene-styrene (SEBS) triblock copolymer system may offer many advantages in this regard. This is a commercial, highly pure material with excellent strength, fatigue, and creep characteristics and is well suited to the fabrication of cardiovascular devices. We propose to examined in detail the SBS and SEBS triblock materials with respect to fabricability and processability for cardiovascular device application and with respect to blood-materials interactions as a function of composition and micromorphology, including surface modifications produced by the hydroxylation of the butadiene domains.